[Virtio-fs] [PATCH v4 0/2] virtiofsd: Improve io bandwidth by replacing pwrite with pwritev

[piaojun]

>From my test, write bandwidth will be improved greatly by replacing
pwrite with pwritev, and the test result as below:

---
pwrite:
# fio -direct=1 -time_based -iodepth=64 -rw=randwrite -ioengine=libaio -bs=1M -size=1G -numjob=16 -runtime=30 -group_reporting -name=file -filename=/mnt/virtiofs/file
file: (g=0): rw=randwrite, bs=1M-1M/1M-1M/1M-1M, ioengine=libaio, iodepth=64
...
fio-2.13
Starting 16 processes
Jobs: 16 (f=16): [w(16)] [100.0% done] [0KB/886.0MB/0KB /s] [0/886/0 iops] [eta 00m:00s]
file: (groupid=0, jobs=16): err= 0: pid=5799: Tue Aug 6 18:48:26 2019
write: io=26881MB, bw=916988KB/s, iops=895, runt= 30018msec

pwritev:
# fio -direct=1 -time_based -iodepth=64 -rw=randwrite -ioengine=libaio -bs=1M -size=1G -numjob=16 -runtime=30 -group_reporting -name=file -filename=/mnt/virtiofs/file
file: (g=0): rw=randwrite, bs=1M-1M/1M-1M/1M-1M, ioengine=libaio, iodepth=64
...
fio-2.13
Starting 16 processes
Jobs: 16 (f=16): [w(16)] [100.0% done] [0KB/1793MB/0KB /s] [0/1793/0 iops] [eta 00m:00s]
file: (groupid=0, jobs=16): err= 0: pid=6328: Tue Aug 6 18:22:17 2019
write: io=52775MB, bw=1758.7MB/s, iops=1758, runt= 30009msec
---

This patch introduces writev and pwritev for lo_write_buf(). I tried my
best not doing harm to the origin code construct, and there will be
some *useless* branches in fuse_buf_copy_one() which are hard to judge
if they will be useful in the future. So I just leave them alone
safely. If the cleanup work is necessary, please let me know.

v2
  - Split into two patches
  - Add the lost flags support, such as FUSE_BUF_PHYS_ADDR

v3
  - use git send-email to make the patch set in one thread
  - move fuse_buf_writev() into fuse_buf_copy()
  - use writev for the src buffers when they're alread already mapped by the daemon process
  - use calloc to replace malloc
  - set res 0 if writev() returns 0

v4
  - iterate from in_buf->buf[0] rather than buf[1]
  - optimize the code to make it more elegant

Jun Piao (2):
  add definition of fuse_buf_writev().
  use fuse_buf_writev to replace fuse_buf_write for better performance

Signed-off-by: Jun Piao <piaojun at huawei.com>
Suggested-by: Dr. David Alan Gilbert <dgilbert at redhat.com>
Suggested-by: Stefan Hajnoczi <stefanha at redhat.com>
---
 buffer.c      |   48 ++++++++++++++++++++++++++++++++++++++++++++++++
 fuse_common.h |   14 ++++++++++++++
 seccomp.c     |    2 ++
 3 files changed, 64 insertions(+)
--